The invention relates to a method of coating the bulb wall of a low-pressure mercury vapour discharge lamp with a luminescent layer, which layer comprises at least one luminescent material, in which method a layer of a suspension consisting of a suspension medium containing the luminescent material and a binder is provided, after which the suspension layer is dried through evaporation of the suspension medium.
Suspension mediums used in the known method are, for example, water or butyl acetate. Binders which can be used are, for example, carboxymethylcelluluose, polyethylene oxide, and nitrocellulose.
In the manufacture of fluorescent lamps having only one luminescent layer, the binder is removed from the suspension layer through heating of the bulb in a sintering oven after drying of the suspension layer. The luminescent layer then remains on the bulb wall with good adhesion.
Problems arise in the manufacture of fluorescent lamps having several, for example two, luminescent layers, especially if the same suspension medium and the same binder are used for the first and for the second luminescent layer.
When the second suspension layer is provided after drying of the first suspension layer, the luminescent material and the binder from the first layer are re-suspended in the liquid second suspension layer. The result is that the first layer is completely or partly flushed off during the provision of the second suspension layer, which is unacceptable. This flushing-off effect could be prevented by removing the binder in the first layer by heating in a sintering oven before the second layer is provided. Compared with the situation in which the bulb is not heated in a sintering oven until after the two layers have been provided, however, this entails an additional process step, which is time-consuming and expensive.